1. Field of the Invention
The present invention relates to the field of power electronics. It relates, in particular, to a controlled turn-off power semiconductor device, comprising
(a) a multiplicity of adjacently arranged, electrically parallel-connected unit cells in a semiconductor substrate between an anode and a cathode;
(b) a layer sequence of a plurality of successive layers doped with alternating polarity within each unit cell between the anode and the cathode; and
(c) within each unit cell, a first MOSFET which is arranged on the cathode side and which is provided for turning off the device.
Such a device is disclosed, for example, by a paper by V. A. K. Temple, IEEE Transactions on Electron Devices, vol. ED-33, No. 10, October 1986, pages 1609-1618 as a MOS controlled thyristor (MCT).
2. Discussion of Background
Modern power electronics at present require fast semiconductor devices which can be driven with simple means and which can be used up to the maximum power ranges.
Controllable semiconductor devices for maximum powers are at present the GTO (gate-turn-off) thyristors. In principle, the entire chip area in the GTO is subdivided into a multiplicity of parallel-connected elementary cells. These devices can be turned on and off by means of the gate contact; because of the high gate currents during turning-off, however, an enormous switching complexity has to be accepted in this connection for the gate drive.
For some years, the development of MOS-controlled devices has been increasingly promoted in power electronics. The advantage of said MOS-controlled devices is based chiefly on the high input impedance at the control electrode. It makes it possible to drive the device with a comparatively very low power consumption.
This trend was initiated by the unipolar power MOSFETs with DMOS structure. Said DMOSFETs have, however, a serious disadvantage: because of the unipolar conduction nature, high breakdown voltages have to be paid for in these devices with high forward resistances which limit the maximum current level.
Higher switchable powers are known to be achievable only with bipolar structures (for example, thyristors). Nevertheless, in such bipolar structures the simple and, in particular, low-power control used in the power MOSFETs should if possible be retained.
It has therefore been proposed to realize the concept described for controlling power semiconductor devices via MOS gates even in devices of the highest power class, namely in thyristors (in this connection see the paper by V. A. K. Temple) mentioned in the introduction.
In such an MOS-controlled thyristor or MCT (MOS Controlled Thyristor) which, like the GTO, comprises a multiplicity of adjacently situated, parallel-connected unit cells, the turning-off is achieved by short-circuiting the emitter to the p-type base through switchable emitter shorts. In this case, MOSFETs which are integrated with the emitter and which may naturally optionally be designed as n- or p-channel MOSFETs are used as switches.
In finely structured thyristors of the MCT or GTO type, however, extremely inhomogeneous current density distributions (filaments) may occur during turning-off (in this connection see K. Lilja and H. Gruning, "Onset of Current Filamentation in GTO Devices", PESC '90, Power Electronics Specialist Conference, pages 398-406 (1990)). In this connection, a small voltage increase in the p-type base layer results in an exponential growth in the electron injection from the cathode. Under normal conditions, this inevitably results in a redistribution and a filamentation of the current, which may destroy the device.